(a) Field of the Invention
The present invention relates to a reference voltage generating device that generates a reference voltage for controlling operation of a control device and a light emitting diode (LED) light emitting device using the control device, a control device including the reference voltage generating device, and an LED light emitting device including the reference voltage generating device.
(b) Description of the Related Art
A light emitting device can be used for displaying an image, or can be used as a light source of a display device such as a liquid crystal display (LCD). Particularly, a light emitting device that is formed with a plurality of LEDS is widely used as a back light of an LCD. The LED light emitting device includes a plurality of LED channels in which a plurality of LED elements are arranged in series and includes a DC/DC converter that supplies an output voltage to each of the plurality of LED channels. An output voltage is supplied to one ends of each of the plurality of LED channels, and a constant current source that controls a current flowing to each of the plurality of LED channels is connected to the other end thereof. The constant current source includes a sync current source and is connected to each of the plurality of LED channels to constantly sustain a current. When a current flows to an LED element and thus light is emitted, a voltage drop occurs in a current direction of both ends of LED. Therefore, the DC/DC converter should output a fully high voltage in consideration of a voltage drop of each LED channel.
However, in view of characteristics of an LED element, voltage drops of LED elements are not constant. If an output voltage of the DC/DC converter is smaller than a voltage drop of each LED channel, a current flowing to a constant current source becomes smaller than a desired value or does not flow. If an output voltage of the DC/DC converter is larger than a voltage drop of each LED channel, power loss increases in the constant current source. Therefore, in order to optimize the output of the DC/DC converter, a headroom controller is used. A voltage of a plurality of LED channels that are detected at an end portion of a plurality of LED channels is a voltage that subtracts an LED voltage drop of each channel from a DC/DC converter output voltage. That is, as LED voltage drop increases, a channel voltage is lowered. The headroom controller detects voltages of each of a plurality of LED channels and selects a minimum value of the detected voltages. The selected minimum value is fed back to the DC/DC converter. That is, when a current continues to flow in all channels, the headroom controller controls an output voltage of the DC/DC converter by feeding back an LED channel voltage having a largest voltage drop.
A method of adjusting brightness of an LED element includes an analog method and a pulse width modulation method. The analog method is a method of adjusting the magnitude of a current continuously flowing to the LED element, and the pulse width modulation method is a method of adjusting a pulse width of a discontinuously flowing current having a predetermined magnitude. In the analog method, when a current flowing to the LED element is small, a color coordinate changes, but in the pulse width modulation method, because a color coordinate does not change, the pulse width modulation method is mainly used. A pulse width modulation method of an LED light emitting device including a plurality of LED channels includes a no delay pulse width modulation method of simultaneously flowing a current of all LED channels and a phase shift pulse width modulation method of allowing a current of each LED channel to have a predetermined delay time and to sequentially flow.
Because the phase shift pulse width modulation method has a relatively smaller load change than that of the no delay pulse width modulation method, an output voltage pulsation of the DC/DC converter is small and a frequency of the pulse width modulation method increases and thus screen noise of the LCD reduces. Further, because a load change is small and a frequency is high, noise of the DC/DC converter reduces.
When applying the phase shift pulse width modulation method to the LED light emitting device including a plurality of LED channels, because voltage drops of each LED channel are different, whenever channels in which a current flows change, a minimum voltage of the plurality of LED channels changes. Therefore, whenever channels in which a current flows change, the output of the DC/DC converter also changes. When a current flows to a channel having a small voltage drop, the output of the DC/DC converter reduces, and when a current flows to a channel having a large voltage drop, the output of the DC/DC converter increases. Therefore, while the output of the DC/DC converter increases, a current having a value smaller than that of a desired current flows or a current does not flow to a channel having a large voltage drop.
Particularly, when a pulse width modulation frequency is high and an LED element has low brightness, a current does not flow. In such a case, there is a problem that a contrast ratio of the LED light emitting device is deteriorated. In order to solve such a problem, a voltage of each of a plurality of LED channels is sampled and held to be fed back to the DC/DC converter. By using this method, when a current does not flow to a constant current source of each LED channel or when a current flows to a constant current source of a channel having a low LED voltage drop, an output voltage of the DC/DC converter and an error amplifier is sustained to an output voltage when a current flows to a channel having a largest LED voltage drop. However, even if this method is used, a predetermined delay time occurs in a sampling holding circuit, and thus when a response speed of the DC/DC converter becomes quick, there is a problem that an entire operation becomes unstable.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.